Geiger-mueller counter tube



July 5, 1966 K. NIENHUIS ETAL GEIGER-MUELLER COUNTER TUBE 2 Sheets-Sheet 1 Filed Oct. 6, 1961 FIG. 2

PRIOR ART Straight ANODE TUB E 'FIG. 2 TUBE with Bent Anode FIGJ F IG. 3

FIG. 4 TUBE with Indicated l7-I3 spacing No extension F IG. 4

lmm

INVENTOR anneus NIENHUIS BY KARS 2N DUUBEN.

FIG. 5

GEIGER-MUELLER COUNTER TUBE 2 Sheets-Sheet 2 Filed 001,. 6, 1961 FIG.6

FIG. 7

INVENTOR KORNELIS NIENHUIS KARS VAN DUUREN United States Patent of Delaware Filed Oct. 6, 1961, Ser. No. 143,454 Claims priority, applicatg Netherlands, Oct. 14, 1960,

,sas 5 Claims. or. 313-93 The invention relates to Geiger-Muller counter tubes for the detection of radiation and to devices comprising such tubes.

Geiger-Muller counter tubes usually comprise a cylindrical cathode which surrounds the discharge space, and a cylindrical, usually wire-shaped, anode arranged along the axis of the cathode cylinder. With the older tubes containing a polyatomic gas, for example, a hydrocarbon, or an alcohol, as a quenching gas, or no quenching gas at all, the anode wire had to be thin for the tubes to operate. Substituting a halogen for the polyatomic g-as improved the properties of the tubes, but due to the electron aifinity of the halogen the efiiciency of these tubes is low when the anode diameter is small. The cause of this was the comparatively weak field near the cathode. Radiation incident in the proximity of the cathode, which often occurs since the cathode usually forms part of the wall, produces electrons and ions which are only slightly accelerated in the weak field or recombine quickly. To improve this, the anodes of later tubes were made large to render the field more homogeneous and hence to produce a higher field intensity at the cathode. An additional advantage of a larger anode was that fewer after-discharges occurred, so that the plateau became longer and flatter. A particular embodiment of such a counter tube contained an anode that was partly spherical and arranged inside a cathode formed by a cylinder whose length to diameter ratio was less than 2. For the detection of ,B-radiation, the latter counter is usually provided at one end with a thin window, which is conductive at least on the inner side and is connected to the cathode.

The principal object of the invention is to reduce the dead time of Geiger-Muller counter tubes without degrading any of its other properties.

The Geiger-Muller counter tube according to the invention comprises a discharge space which is filled with an ionizable gas containing at least 0.005% and at the most 25% by volume of a halogen and the remainder of a rare gas, with a cathode surrounding the discharge space. An anode is arranged inside the cathode and is characterized in that the distance between it and the cathode is smaller at a restricted number of places than onequarter of the mean distance between the anode and the cathode, so that during the operation of the tube a field intensity is produced at the cathode at the said places of a considerably higher value than the mean field intensity at the cathode.

From experiments leading to the invention, it has been found that tubes according to the invention have a materially shorter dead time than tubes of similar structure which lack places where the distance from the anode to the cathode is shorter than one-quarter of the mean dis tance between the anode and the cathode. It has, moreover, been found that the starting voltage of tubes according to the invention is lower, and that a longer and flatter plateau is obtained.

The strongly reduced dead time of tubes according to the invention is very surprising and it may probably be explained as follows, though this is not to be considered in any way as limiting. When radiation is incident somewhere in the space between the cathode and the anode, ionisation of the gas occurs locally. The negative electrons produced are accelerated by the electric field in the direction towards the positive anode and produce new ionizations during their passage. In this way, the known avalanche effect is produced. When the electrons finally strike the anode, the tube is ignited. The flow of current in the anode circuit, which usually contains a resistor, reduces the voltage between anode and cathode, which terminates the electron multiplication process and ultimately the discharge. The time duration of this whole process of build up and disappearance, which predominates, of the discharge depends upon many factors, but is determined mainly by the field intensity at the cathode. The avalanche formation by the incident radiation involves a further phenomenon which is particularly important in explaining the operation of counters according to the invention. During the avalanche formation process, also photons are emitted, which travel with great velocity through the discharge space and thus reach, in tubes according to the invention, very soon the strong field at the cathode at the aforesaid places, obtained by reducing the distance between the anode and cathode. In this stronger field, the photons also produce ionization, and since the field is so strong in situ, both the growth of the ionization current and mainly the disappearance thereof occur very rapidly. It is found that when the aforesaid requirement for the distance between the anode and the cathode to be reduced at a small number of places to less than one-fourth of the mean distance between the anode and the cathode is fulfilled, the final discharge in the tube takes place virtually only at the areas concerned. Consequently, the time required by the photon, whether of the first or a later generation, to travel from its place of origin to a place of increased field intensity, in addition to the time required for the production of the discharge in situ, is short compared with the build up time of the discharge in regions with Weaker fields. Once the discharge has been produced in the region of higher field intensity, the voltage between the anode and the cathode drops to a value at which no further discharges can occur at any other places. The dead time of the counter is then mainly determined by the disappearance of the positive space charge in the localized region of high field intensity. This disappearance occurs very rapidly owing to the high field intensity; this means, consequently,,that the dead time is short. Counters could be constructed with an intensive field between the anode and the cathode, for example, by using flat sheets or concentric cylinders as anode and cathode, arranged at a very small distance from each other. The capacity of such tubes, however, becomes very high and this produces a long dead time. Consequently, as stated above, only at a restricted number of locations is the distance between anode and cathode to be made small. For satisfactory results, the total anode surface area spaced the shorter distance from the cathode must be less than one-tenth of the total active anode surface capable of participating in the discharge. In principle, it is not important how the reduction of the distance is obtained; either the cathode or the anode may be arranged locally nearer the anode or the cathode respectively. For practical reasons, the anode will, as a rule, be shaped in a particular form to achieve this result. Also, it is important that the localized regions of high intensity lie adjacent to the counter wall portion through which the radiation can enter.

It is not necessary for the cathode to form part of the wall of the discharge tube, but usually counter tubes according to the invention will be constructed in this way for practical reasons. Counters according to the invention may be provided with a thin window transparent to beta-rays. With a cylindrical counter preferably the window is provided at one end of the cylinder, and the localized region of high field intensity provided adjacent the window.

Several embodiments of counter tubes according to the invention are possible. For example, if the cathode is a cylinder whose length to diameter ratio exceeds 2, an anode which is predominantly straight may be arranged inside the cathode. This anode, which may be a solid or a hollow cylinder, is provided at least at one place with a kink or bend so that the distance between the kinking point and the cathode is smaller than one-fourth of the distance of the straight portion of the anode to the cathode. A further modification involves a cylindrical, for example wire-shaped, anode which is bent in zigzag fashion. As a further alternative, a straight anode may be provided with radially-extending conductive members which terminate close to the cathode surface to provide the 10- calized regions of high field intensity.

With counter tubes according to the invention in which the cathode is a cylinder whose length to diameter ratio is less than 2, the anode may be constructed in the form of an at least partly spherical body with at least one extension or projection directed towards the cathode. If the tube, as described above, is provided with a window, preferably the extension is orientated towards the window, since the radiation to be detected, particularly the i-radiation, is usually incident from that side.

The invention will now be described with reference to the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of a cylindrical counter tube comprising a wire-shaped anode having one kink;

FIG. 2 is a cross-sectional view, to scale, of a counter tube having a zigzag anode;

FIG. 3 is a graph of dead time versus tube voltage for counter tubes;

FIG. 4 is a cross-sectional view, to scale, of a counter tube having a hemispherical anode and a window;

FIG. 5 is a graph similar to that of FIG. 3 for a tube as shown in FIG. 4;

FIG. 6 is a cross-sectional view of a modification of a counter tube having a spherical anode;

FIG. 7 is a diagrammatic view of a device comprising a counter tube as shown in FIG. 4.

In the graphs of FIGS. 3 and 5, the voltage between the anode and the cathode is plotted along the abscissa and the dead time, expressed in microseconds, is plotted along the ordinate. All measurements were carried out on tubes having a gas mixture consisting of 0.1% by volume of argon, 0.1% by volume of bromine and 99.8% by volume of neon. Of course, it is understood that other rare gasses, alone or in combination, may be employed, that the halogen proportions can be varied over a considerable range between 0.005 and 25%, though the smaller values are preferred, and that chlorine may 'be used in place of the bromine.

The counter tubes shown in the figures may be manufactured from conventional materials, for example, ferrochromium for the conductive parts and glass for the insulating parts sealed directly to the metal. After assembly and thorough cleaning of the metal parts, the tubes are evacuated and then filled with a mixture of the rare gas and the halogen, which mixture fulfills the aforesaid conditions. The total gas pressure depends upon the intended use of the counter and lies, as a rule, between and ms. Hg. The wall thickness is, of course, chosen in accordance with the penetration power of the radiation to be measured.

Referring now to FIG. 1, a cylindrical cathode 1 encloses a mainly straight anode 2 along its axis, which anode is fastened in the cathode by means of a glass head 3. The radiation in this case enters the tube through the cathode 1. The anode 2 has a V-shaped bend or kink 4, of which the top or peak is located so near the cathode 1 that the distance in situ is smaller than one-fourth of the distance 5 between the straight portion of the anode 2 and the cathode 1. The kink need not be sharp; in general, it is even advisable to round off the peak of the kink.

FIG. 2 shows a similar tube comprising a cylindrical cathode 6 and anode 7, which is fixed in the cathode 6 in an insulated manner by means of a glass head 8. The anode 7 is bent in a zigzag fashion, so that four places are formed where the distance between the anode and the cathode is very small and hence the field intensity at the cathode is very high. When a voltage of 600 v. is applied between the anode and the cathode a field intensity of about 3000 v./crn. at the cathode and of 6000 v./cm. at the anode was calculated at the kinking points. For comparison, the field intensity was calculated for a similar voltage with a corresponding tube having a completely straight anode wire. At the anode, the value of the field intensity was found to be 2900 v./cm. and at' the cathode to be 350 v./cm.

In the graph of FIG. 3, the curve 9 designates the variation in dead time as a function of the anode voltage for the tube shown in FIG. 2, measured with a quenching resistance of 2M ohms. By way of comparison, the dead time was measured for a corresponding tube having a completely straight anode wire. The dead time found in the latter case as a function of the anode voltage is indicated in FIG. 3 by the curve 11. It is clearly evident that this particular embodiment according to the invention provides a material reduction of the dead time.

The tube shown in FIG. 4 comprises a cylindrical wall 12 constituting at the same time the cathode of the counter tube. At one end the counter is closed off by a thin window 13, pervious to fi-rays, consisting of ferrochromium with a weight of 10 mgs./cm. and connected conductively to the cathode 12. Reference numeral, 14 designates 'a hemispherical anode, which is fastened in the cathode 12 by means of its supply wire 15 and insulating material 16. The anode 14 has a projection or extension 17 directed towards the window 13. The distance of the end of the extension 17 from the window 13 is very small and amounts to less than one-fourth of the mean distance of the surface of the sphere 14 from the cathode 12, 13. Instead of using a ferrochromiurn window, use may be made of an insulating window, for example, of mica, which is rendered conductive on its inner side, for example by applying a thin metal layer by vaporization.

At a voltage of 600 v. between the anode 14 and the cathode 12, '13, both at the anode and at the cathode a field intensity was calculated between the end of the extension 17 and the window 13 of about 8000 v./cm. At the region 10, a field intensity Was calculated between the sphere and the cathode at the anode of 1700 v./cm. and at the cathode of 400 v./cm.

In FIG. 5, the curve 20 illustrates the relationship between the anode voltage and the dead time of the counter tube shown in FIG. 4, measured with a quenching resistor of IBM ohms, wherein the spacing between the extension 17 and the window 13 was '1 mm. By way of comparison, curve 18 exhibits the same relationship for a distance of 3 mrns. between the extension 17 and the window 16; curve 19 was determined for a distance of 1.9 mms. The curve 21 applies to a counter tube as shown in FIG. 4 in which the anode does not contain the extension 17 and the distance between the anode and the window is 5.4 unms. It is evident from this graph, again, that the dead time in considerably improved in tubes according to the invention. A result comparable to that obtained with the embodiment illustrated in FIG. 4 can be achieved by eliminating the extension 17 and instead positioning the anode 14 so close to the window 13 that a single region of high field intensity is formed between the bottom tip of the spherical anode 14 and the opposed window portion. In this case, the spacing must be very small, for example, about 1 mm., which would thus be substantially less than one-quarter of the average spacing of the active anode surface to the shell 12. But this configuration has a disadvantage, because the counting efliciency in the center of the window will be decreased somewhat. This is due to the vfact that the electrons transversing the center part of the window will have only rather small path lengths in the counter gas volume and the detection efliciency will therefore be decreased. The eifect is present in the configuration of FIG. 4 to a much smaller extent.

FIG. 6 shows a further embodiment of a counter tube according to the invention comprising a cylindrical cathode 22, closed-off at the bottom, and a spherical anode 23 having four (one of which is not shown) conductive extensions 24 extending towards the cathode 22 to provide four regions of high-field intensity. The incident radiation enters this counter directly through the oathode walls.

FIG. 7 shows diagrammatically a device for counting B-rays by means of an end Window counter tube as shown in FIG. 4. -In this figure, 133 designates the short, cylindrical cathode of the counter and 34 the hemispherical anode with a current supply wire 35. This current supply wire is connected via quenching resistors 36 and 37 to the positive terminal of a voltage source '38, of which the negative terminal is connected to the cathode 33. The pulses produced by the radiation from a specimen 39 are transmitted via the capacitor 40 to a counting unit 41. As is conventional with Geiger-Muller tubes, the voltage applied to the tube is selected to have a value along the plateau region of its counting rate-voltage discharge characteristic. As a rule, the voltage is selected at about the center of the plateau region.

The structure of the counter illustrated in FIG. 7 and operated as a Geiger counter not only offers the advantage of a remarkably low dead time, but its counting efficiency remains high. This is due, among other things, to the \high rfield intensities existing everywhere adjacent the end window, through which the radiation enters the tube. This means that ion-pairs wherever created within the counter in the vicinity of the window will have a high probability of initiating an avalanche, resulting in a current pulse in the output circuit.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this :art without departing from the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

1. A Geiger-Muller counter tube for detecting radiation comprising a cylindrical cathode electrode whose length-diameter ratio is less than 2 and surrounding a discharge space containing an ionizable gas filling constituted of a rare gas and between 0.005% and 25% by volume of a halogen quench, said cathode including a radiation-pervious window portion, an anode electrode disposed within the cathode and having an active spherical anode surface defining wth the opposed active cathode surface a mean spacing, said anode having a conductive portion extending toward the said window portion and spaced therefrom a distance less than one-fourth of the said mean spacing, .and means for applying .a potential diflerence between the cathode and anode, whereby at the said extension a localized region of high field intensity is established thereby reducing the dead time of the tube.

2. A counter tube as set forth in claim 1 wherein the window portion is pervious to B-rays and is conductively connected to the cathode.

3. A counter tube as set forth in claim 1 wherein the conductive portion constitutes a rod-like member connected to the spherical anode portion and extending close to but spaced from the cathode window portion.

4. A Geiger-Muller counter tube for detecting radiation comprising a generally cylindrical cathode electrode iwhose length-to-diameter ratio exceeds 2 and surrounding a discharge space containing an ionizable gas filling constituted of .a rare gas and between 0.005% and 25% by volume of a halogen quench, said cathode including a radiation-pervious portion through which the radiation to be detected can enter the discharge space, a generally wire-like anode electrode extending mainly along the axis of the cathode and having an active anode surface defining with the opposed active cathode surface a mean spacing, said anode having an intermediate portion bent off the axis toward the active cathode and .spaced therefrom a distance less than one-fourth of the said mean spacing, the total anode surface at the said bent intermediate portion being less than one-tenth of the total active anode surface, and means for applying a potential difference between the cathode and anode in the plateau region of its discharge characteristic, whereby at the said bent intermediate portion a localized region of high field in tensity is established thereby reducing the dead time of the tube.

5. A counter tube as set forth in claim 4 wherein the anode is a wire which extends in zig-zag form within the cathode providing several localized regions of high field intensity.

References Cited by the Examiner UNITED STATES PATENTS 2,5 52,723 5/ 1951 Koury -3 l393 2,776,390 1/1957 Anton 3l3-93 2,917,647 12/:1959 Fowler et al. 313-93 2,944,179 7/ 1960 Lafferty 3 1393 X 3,056,059 9/ 1962 Herrnsen et al. '3'l-393 HERMAN KARL SAALBACH, Primary Examiner.

JOHN W. HUCKERT, Examiner.

S. .CHATMON, ]R., Assistant Examiner. 

1. A GEIGER-MUELLER COUNTER TUBE FOR DETECTING RADIATION COMPRISING A CYLINDRICAL CATHODE ELECTRODE WHOSE LENGTH-DIAMETER RATIO IS LESS THAN 2 AND SURROUNDING A DISCHARGE SPACE CONTAINING AN IONIZABLE GAS FILLING CONSTITUTED OF A RARE GAS AND BETWEEN 0.005% AND 25% BY VOLUME OF A HALOGEN QUENCH, SAID CATHODE INCLUDING A RADIATION-PERVIOUS WINDOW PORTION, AN ANODE ELECTRODE DISPOSED WITHIN THE CATHODE AND HAVING AN ACTIVE SPHERICAL ANODE SURFACE DEFINING WITH THE OPPOSED ACTIVE CATHODE SURFACE A MEAN SPACING, SAID ANODE HAVING A CONDUCTIVE PORTION EXTENDING TOWARD THE SAID WINDOW PORTION AND SPACED THEREFROM A DISTANCES LESS THAN ONE-FOURTH OF THE SAID MEAN SPACING, AND MEANS FOR APPLYING A POTENTIAL DIFFERENCE BETWEEN THE CATHODE AND ANODE, WHEREBY AT THE SAID EXTENSION A LOCALIZED REGION OF HIGH FIELD INTENSITY IS ESTABLISHED THEREBY REDUCING THE DEAD TIME OF THE TUBE. 